I have experienced the same problem with the 49g+. John is right, turn off the FLAG that displays the clock.
Also, I found that when there are more than 3 objects on the stack, I still get missed keystrokes. So, if you must use the machine, consider clearing the stack often ( or write a routine to do it auto?)
Anyway, it does not appear to be a mechanical issue.
I have called hp calc tech sppt (1800hpinvent) and after some verbal diagnostics, I was told that a replacement would be shipped out. I'm currently waiting for it.
However, since I'm taking the FE exam this October, I will probably continue using only the 33s to get fluent on it.
Good luck,
Eric